Clickers are the zombies that have been infected by the mushrooms, the source of the infection for a long time. they are blind, but can hear very well. they make clicking noises to navigate, much like bats. they are just sons of bitches.

These zombies are called clickers. The post makes fun of their name, by showing other 'clickers' like a lightswitch, mouse, clickypen and remote. By the way, the zombie biting him in the last panel is indeed a clicker.